1. Field of the Invention
The present invention relates to a coupler employing dielectric lines and more particularly to a dielectric rotary coupler effective in transmitting an electric signal to a rotating member or in receiving an electric signal from a rotating member.
2. Description of the Prior Art
It is known that, where two transmission paths formed of dielectric lines, or waveguides, are disposed closely to each other, if a signal is supplied to one of the dielectric lines, energy of the signal propagated along that dielectric line is coupled into the other dielectric line (refer to Institute of Electronics and Communication Engineers of Japan Technical Research Report: Microwave, Volume 18, No. 93, 1981.7.24, MW8137).
The phenomenon will be described in detail in the following.
In FIG. 5 is shown an example of a line made of a dielectric (relative dielectric constant .epsilon..sub.1). The dielectric line in a rectangular section (a, b) is placed in a medium (including air) having a lower relative dielectric constant .epsilon..sub.2 than that of the same .epsilon..sub.1.
If, now, an electromagnetic wave of the class of a microwave or millimeter wave (in the frequency range between 1 GHz and hundreds GHz), having electric power P.sub.1 is input to the dielectric line (hereinafter to be simply called line) 1 from its one end 1a, the electromagnetic wave can be confined in the line 1, propagated along the Z axis, and taken out from the side of the terminal 1b as power P.sub.2.
At that time, even if the dielectric line 1 is bent, the electromagnetic wave travels along the line 1.
The mode of the electromagnetic wave propagating in the line 1 varies with the frequencies of the input signal, the sectional forms and dimensions of the line 1, the relative dielectric constants of the medium .epsilon..sub.2 surrounding the line 1 whose relative dielectric constant is .epsilon..sub.1, and so forth. When these are set at suitable values, the transverse mode of the electromagnetic wave propagating along the line 1 can be made into a single propagating waveform.
And the propagation wavelength can be set on the order of some centimeters to 0.1 mm.
Now, a coupler formed of such lines will be described in the following.
A second line 2 formed of a dielectric is disposed in parallel with a first line 1 at a distance of d.sub.1 as shown in FIG. 6.
When an electromagnetic wave whose power is P.sub.1 is input to the first line 1 from its one end 1a, it travels along the Z axis as described above. But in the case where the second line 2 is disposed at the position Z=Z.sub.1, the electromagnetic wave (shown with fine lines) which has been propagated up to this point begins now to be coupled into the second line 2. This phenomenon of coupling, which depends upon the changes in the propagation mode as will be described later, could be considered to be gradual penetration of the electromagnetic wave traveling along the first line 1 into the second line 2. Power P.sub.2 which is coupled into the second line 2 reaches its maximum value at the point Z=Z.sub.2 and, as the electromagnetic wave travels further, it is reversely coupled from the second line 2 into the first line 1, and thus the most of the power P.sub.2 is returned to the first line 1 at the point Z=Z.sub.3.
In this case, Z.sub.2 -Z.sub.1 =Z.sub.3 -Z.sub.2 =L.sub.0 is designated a coupling length of the dielectric lines.
Such transition of energy of an electromagnetic wave as described above is caused by the difference in phase constants of the propagating wave of an even mode and that of an odd mode.
If it is assumed, for example, that one dielectric line is formed of the first line 1 and the second line 2 as shown in FIG. 7, then two modes, i.e., an even mode wave S and an odd mode wave A, are considered to be traveling in vibrating motion.
Then, the above mentioned coupling length L.sub.0 is given by: EQU L.sub.0 =.pi./(.beta..sub.zS -.beta..sub.zA),
where .beta..sub.zS is the phase constant of the even mode wave S in the direction of the Z axis and .beta..sub.zA is the phase constant of the odd mode wave A in the direction of the Z axis.
Now, in order to maximize the electromagnetic energy coupled from the first line 1 into the second line 2, the two lines may be arranged such that the portion overlapping each other becomes the coupling length L.sub.0. However, if the second line 2 is bent at a sharp angle at the end of the coupling length L.sub.0 or cut off there, the propagation mode of the electromagnetic wave is disturbed at this point and a satisfactory result cannot be obtained.